If you never saved it explicitly to another location, it should still be in your user directory's Adobe Illustrator sub-branch, if you haven't deleted it. You could then load it as a custom external swatch in the swatches palette. Newer versions never automatically import older data as all directories are kept separate.

Thank you for the thank you. One thing I just thought of that might also help somebody – AppData is normally a hidden folder, so a person may not see it until they set things to Folder Options, View, Show Hidden Files. I have a habit of always setting this back to "Don't show" as soon as I'm done; I learned that one time, the hard way.